1. Field of the Invention
The present invention relates generally to removing impurities from amine absorbing solutions for acidic gas in gas sweetening systems. More specifically, the present invention provides a method for removing sludge from amine absorbing solutions by contacting a stream of regenerated absorbing solution with CO.sub.2.
2. Description of the Prior Art
It is well known in the art to treat gases and liquids with amine absorbing solutions to remove acidic gases such as CO.sub.2 and H.sub.2 S. The amine usually contacts the acidic gases and liquids as an aqueous solution containing amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The acid gas scrubbing processes known in the art can be generally broken into three (3) categories.
The first category is generally referred to as the aqueous amine process where relatively concentrated amine solutions are employed during the absorption. This type of process is often utilized in the manufacture of ammonia where nearly complete removal of the acid gas, such as CO.sub.2, is required. It is also used in those instances where an acid gas, such as CO.sub.2, occurs with other acid gases or where the partial pressure of the CO.sub.2 and other gases are low.
A second category is generally referred to as the aqueous base scrubbing process or "hot pot" process. In this type of process, a small level of an amine is included as an activator for the aqueous base used in the scrubbing solution. This type of process is generally used where bulk removal of an acid gas, such as CO.sub.2, is required. This process also applies to situations where the CO.sub.2 and feed gas pressures are high. In such processes, useful results are achieved using aqueous potassium carbonate solutions and an amine activator.
A third category is generally referred to as the nonaqueous solvent process. In this process, water is a minor constituent of the scrubbing solution and the amine is dissolved in the liquid phase containing the solvent. In this process, up to 50% of the amine is dissolved in the liquid phase. This type of process is utilized for specialized applications where the partial pressure of CO.sub.2 is extremely high and/or where many acid gases are present, e.g., COS, CH.sub.3 SH and CS.sub.2.
In the known absorption operations, the enriched absorbent or solvent, after being withdrawn from the absorber column, is passed into a regenerator column where the absorbed H.sub.2 S and/or CO.sub.2 are stripped by countercurrent contacting with steam. The steam is generated at the bottom of the regenerator column by boiling the solution in an indirectly heated tubular heat exchanger or "reboiler". The heat for this boiling may come from steam, or from any hot fluid, or from direct firing. The countercurrent contacting with steam in the regenerator column, followed by boiling, will strip the H.sub.2 S and or CO.sub.2 from the solution down to a very low residual level. The hot stripped solution, known as "lean solution", or regenerated absorbing solution, is withdrawn from the reboiler, cooled, and sent back to the absorber column to complete the cycle of absorption and regeneration.
It has been found, however, that acid gas absorbing solutions can react with elemental sulfur to form dissolved solids (sludge) at temperatures normally found in gas plants. Unless elemental sulfur can be excluded from the system, the sludge will be a permanent feature of gas treatment with amine absorbing solutions. Filtering the sludge from absorbing solution that is heavily loaded with H.sub.2 S (rich absorbing solution) is hazardous. Consequently, there exists a need for an alternative method to remove sludge from acid gas absorbing solutions in gas sweetening systems.